In recent years, attention has been focused on methods of high heat flux removal at low surface temperatures. This is due in large part to the advancing requirements of the electronics industry that prevent high temperature heat transfer due to the operating conditions of electronics. Though the heat transfer process is very complex and still not completely understood, many evaporative spray cooling experiments have been performed which indicated the high heat removal capability of this cooling technique. The spray technique generally works in the following way; a spray nozzle is used to atomize a pressurized liquid, and the resulting droplets are impinged onto a heated surface. A thin film of liquid is formed on the heat transfer surface in which nucleate boiling takes place. The droplet impingement simultaneously causes intense convection and free surface evaporation. When a liquid with a high latent heat of vaporization (such as water) is used, over 1 kW/cm2 of heat removal capability has been demonstrated.
The temperature of the cooled surface is determined by the boiling point of the liquid. Since the resulting heat transfer coefficient is very large (50,000 to 500,000 W/m2C) the surface temperature will be only a few degrees centigrade above the boiling point of liquid.
This type of cooling technique is most appropriately implemented when used to cool high heat flux devices such as power electronics, microwave and radio frequency generators, and diode laser arrays.
Prior art describes processes and devices related to cooling of small, individual electronic chips. This can be seen in, for example, U.S. Pat. Nos. 5,854,092; 5,718,117; and 5,220,804. This prior art uses a liquid spray to cool individual electronic components, or an array of these individual components located at discrete distances from each other. Since the electronic components (the heat sources) are individual devices with spaces between, the liquid spray cones do not overlap or interact with each other. The typical size of an electronic chip is 2 cm2 in area and is spaced at a distance of 0.5 to 1 cm. This allows the prior art to cool these chips with an impinging spray without interfering with the spray process of the surrounding chips.
As stated above, diode laser arrays and microwave generators are devices that can be cooled with this type of impinging spray technology. Current market forces are driving these devices to increased power and size requirements. As a result, high heat flux devices are now being designed with surface areas much larger than 2 cm2. New high heat flux devices will be 100 cm2 to 1000 cm2. The entire large surface area will need to be cooled at the same high heat flux rate as the small devices were in the prior art. However, the prior art does not detail a method to cool such a large device. Prior art only details a method to cool several small individual devices.
It may be thought that a large surface could be cooled with an array of nozzles spraying down on the large surface in the same way a single nozzle sprays down on a small surface, as shown in the prior art. However, it has been shown in a study with air jet impingement that scaling in this way is not possible. Simply put, the effectiveness of the jets or sprays in the center of the array interact with each other in a way that considerably reduces the ability to transfer heat. This is a result of the fluid flow accumulating as the fluid moves outward from the stagnation point. A good portion of the impinging droplets are vaporized with this system, however, this is not so for all the liquid. The remaining liquid will flow off the heated surface and be returned to the pump. When the surface is large, the fluid from the nozzles at the center of the surface will need to travel across the entire surface before exiting at the edges. This can be called the xe2x80x9cspray liquid run-off problem.xe2x80x9d
The subject invention pertains to a method and apparatus for high heat flux heat transfer. The subject invention can be utilized to transfer heat from a heat source to a coolant such that the transferred heat can be effectively transported to another location. Examples of heat sources from which heat can be transferred from include, for example, fluids and surfaces. The coolant to which the heat is transferred can be sprayed onto a surface which is in thermal contact with the heat source, such that the coolant sprayed onto the surface in thermal contact with the heat absorbs heat from the surface and carries the absorbed heat away as the coolant leaves the surface. The surface can be, for example, the surface of an interface plate in thermal contact with the heat source or a surface integral with the heat source. The coolant sprayed onto the surface can initially be a liquid and remain a liquid after absorbing the heat, or can in part or in whole be converted to a gas or vapor after absorbing the heat. The coolant can be sprayed onto the surface, for example, as a stream of liquid after being atomized, or in other ways which allow the coolant to contact the surface and absorb heat. Once the heat is absorbed by the coolant, the coolant can be transported to another location so as to transport the absorbed heat as well.
The subject invention pertains to a method and apparatus for cooling surfaces and/or devices. In a specific embodiment, the subject invention can incorporate a spray nozzle and a cooling/electronic interface surface. The spray nozzle may use pressurized liquid (commonly known as pressure atomizer nozzles), pressurized liquid and pressurized vapor (commonly known as vapor assist nozzles), and/or pressurized vapor nozzle (commonly known as vapor blast or vapor atomizer nozzles) to develop the atomized liquid spray used in the cooling process.
In a specific embodiment, the cooling/electronic interface surface can be compartmentalized such that spray entering one compartment is impeded from crossing over to adjacent compartments. In a further specific embodiment, a plurality of nozzles can each spray into one of a plurality of compartments such that spray from each individual nozzle is applied to a specific target area. For example, each nozzle may spray one compartment. The excess liquid which enters each compartment can then be forced out of the compartment in a counter-parallel flow from the spray direction rather than a perpendicular flow as in prior art, so as to correct the liquid run-off problem. The shape and depths of the compartments can vary according to the type of nozzle used to atomize the liquid coolant. Preferably, the subject compartments incorporate side walls which can redirect the exiting flow in a pattern that is not perpendicular to the incoming flow.
The atomized spray can be directed onto the rear surface of the compartmentalized interface plate. The spray is preferably positioned to create the most even application of atomized liquid onto the entire rear surface. The liquid can be sprayed at a temperature near its boiling point. Thus, when the liquid hits the heated surface in the rear of the compartment, the liquid can begin to boil. The heat from the electronics, or other heat source, is transferred through the interface into the boiling liquid spray at a very high rate. The created coolant vapor and excess liquid exit the compartment in a direction that is not perpendicular to the incoming flow. Under the operating conditions of an open loop system, the boiling point of the liquid coolant must be at ambient pressure since the evaporating environment is exposed to the ambient. Under these conditions, the heat removed by the developed vapor is released to the atmosphere. However, not all vaporized coolants can be responsibly released to the atmosphere, due, for example, to environmental concerns. In addition, coolants with boiling points other than ambient may be preferred. Accordingly, specific embodiments of the subject invention can be operated in a closed loop.
In a closed loop system, the interface plate can be located within a sealed housing so that the spray and the resultant vapor is trapped within the sealed housing. Under this condition, the pressure within the housing can influence the boiling point of the coolant and the operating temperature. As the coolant vaporizes, it carries the heat from, for example, electronics, away from the interface plate. Since the system is now closed, the vapor can be condensed and the heat released out of the housing through a condenser. The condenser can incorporate, for example, a standard heat exchanger or can operate via a sub-cooled mist of the coolant sprayed within the housing. The mist can be sub-cooled below the saturation temperature of the coolant within the housing via an external heat exchanger. As the sub-cooled liquid spray contacts the saturated vapor, heat is transferred to the spray and the vapor condenses on the liquid droplets and flows to a liquid reservoir.
The coolant can be drawn from the liquid reservoir, for example, by a liquid pump or via venturi action of a vapor atomizer nozzle. The liquid then flows through the nozzle and is once again sprayed onto the interface plate. The circulation of the coolant within the closed process depends on the type of atomizer used. If pressure atomizer nozzles are used, then a liquid pump can suffice. If vapor assist nozzles or vapor atomizer nozzles are used, then both a vapor compressor and a liquid pump can be used in the circulation of the coolant.
Typically, the heat gained by the liquid in the closed system is transferred to a refrigerant of a vapor compression cycle via a heat exchanger. The vapor compression cycle increases the temperature of the now warmer refrigerant and allows it to release the heat to the environment. This is commonly known as the chiller loop.
An additional feature can be added to the closed system that combines it with a vapor compression cycle without the heat exchanger interface between the two loops. This combination involves using a refrigerant as the coolant in both loops. Under this scenario, liquid refrigerant can be atomized onto the interface plate. Vapor and excess liquid refrigerant can be expelled from the compartment and flow into the housing. The saturated vapor can be removed from the housing with a vapor compressor and can be compressed to a temperature above ambient temperature of the final heat sink, for example atmospheric air. The now superheated vapor can flow through a heat exchanger releasing the heat to the final heat sink. As the heat is released, the superheated vapor condenses to liquid refrigerant. As is common to vapor compression cycles, the higher pressure saturated liquid can flow through an expansion valve. The liquid is allowed to expand to the pressure of the housing, cools to its saturation temperature within the housing, and flows to the liquid reservoir ready to begin the process once again.